In recent years, a torque converter (hydraulic power transmission device) provided between an engine and an automatic transmission mounted on a vehicle is provided with a lockup clutch which performs lockup of the torque converter for eliminating slip loss of the torque converter during running and improving fuel consumption. The lockup clutch moves a piston by supplying oil into a hydraulic oil chamber to thereby engage plural friction plates to perform lockup.
As the above lockup clutch, there is proposed a structure in which an oil passage communicating with a hydraulic oil chamber is formed by coupling a flange member to a center piece of a front cover of the torque converter (Patent Literature 1). In the case of the structure described in Patent Literature 1, the coupling between the center piece and the flange member is performed in the following manner. That is, relative rotation in a circumferential direction is prevented by engaging the center piece with the flange member respectively by claws. Furthermore, the flange member is fit onto a cylindrical portion which is integrally formed with the center piece as well as fixed by a snap ring to thereby regulate positions of the center piece and the flange member in an axial direction.